Connection to Domain controller

Hi all, bet this is a discussed issue but i came across something that I do not understand. I setup my domain controller on a 2k3server. The full domain name is uk.support.com but if I try to connect a other box to the this domain using this full name I can not gives an error that could not find the domain. The wai I get it to work is using the label Domain name (pre-win2000) which is UK only. In this way it connects. Actually i notice that if I check the local users on the DC i have UK\Administrato… why is not uk.support.com..??

Thanks all
larbac